Hi halbo5
The possible last names are controlled in the global settings in the "User" tab.
Your screenshot looks to me "a little" as if you have set the control type to "Allow" but the plugin can not split the line breaks from the field below ... weired ... never have seen this before
Is your Joomla installation on a windows or linux server? And what happens if you change the last name control setting to e.g. "None"?
Greetz
FoTo50